This competing renewal proposes to evaluate the impact of recent Mexican drug policy reform on HIV-associated risk factors and protective factors among injection drug users (IDUs) in Tijuana, Mexico. In Aug/2009, Mexico approved a law that partially deregulates possession of small, specified amounts of cocaine, heroin, methamphetamine and marijuana for personal use. The law specifies that after this law is enacted in Aug/2010, police apprehending persons who possess sub-threshold amounts of any of these drugs will be released with a record noting that they received 'no penal action'; however, upon a third apprehension, they will be required to enter drug treatment, or go to jail. States are required to have completed drug treatment expansion by Aug/2012. The law has been met with praise and sharp criticism about potential unintended consequences. Our binational research team, which has conducted studies among IDUs in Tijuana for 6 years, proposes a longitudinal, mixed methods study to address the impact of Mexican drug policy reform on the following outcomes among IDUs in Tijuana: 1) Changes in knowledge, attitudes and experiences about the new law and their relationship to drug using behaviors and treatment readiness; 2) Temporal trends in drug use behaviors: i) frequency of injection, cessation and relapse; ii) possession and use of specific drugs and combinations; iii) involvement in gangs and drug economy; 3) Health risks and protective behaviors: i) injecting in shooting galleries and public spaces, ii) receptive needle sharing, iii) NEP and pharmacy attendance, iv) prevalence and incidence of overdose, HIV infection and death; 4) Experiences with drug treatment: i) proportion of drug users over time choosing treatment over incarceration; ii) incidence and experiences with voluntary and court-mandated drug treatment; 5) Law enforcement practices and interactions with IDUs: i) rates of arrest and incarceration; ii) perceived reasons vs. recorded reasons for arrest and incarceration; ii) experiences of cohort participants with police, including corruption and abuse. We will recruit a cohort of 750 HIV-negative IDUs through respondent driven sampling who will undergo semi- annual quantitative interviews and HIV tests. At each visit, a sub-sample who receive citations for drug possession-- especially those entering drug treatment involuntarily -- will be selected to undergo qualitative in- depth interviews to provide context on their knowledge of the law (Aim 1), experiences with drug treatment (Aim 4) and police (Aim 5). Administrative records from drug treatment programs, the police department and jails/prisons will be obtained to serve as independent outcome measures and assess city-level trends. Apart from Mexico, other Latin American countries and elsewhere are adopting more relaxed drug policy reforms. Given Mexico's important role in production and trafficking of illicit drugs, and Tijuana's location as a major corridor through which illicit drugs enter the U.S., this study represents an unprecedented natural experiment, allowing us to evaluate impacts of relaxed drug policy reforms on drug use, treatment, and HIV risk behaviors.